In recent years, there have been progressed development of image display devices having a function of a transparent display which not only displays an image but also enables the background to be transparently visible. For example, in Japanese Patent Application Laid-Open No. 2010-91609, there is disclosed a configuration of a transparent display that enables a display image to be easily visible by making the background opaque when displaying the image. FIG. 53 is a diagram showing a configuration of a liquid crystal display device 800 having the function of the transparent display disclosed in Japanese Patent Application Laid-Open No. 2010-91609. As shown in FIG. 53, the liquid crystal display device 800 includes a liquid-crystal panel 810, a shutter film 820 provided at the back surface of the liquid-crystal panel 810, and a control unit 830 that controls the drive of the liquid-crystal panel 810 and the shutter film 820. The shutter film 820 switches between a state of directly passing the incident light therethrough to radiate the liquid-crystal panel 810 and making the rear side of the shutter film 820 distinguishably transparent and a state of indirectly passing the incident light therethrough to radiate the liquid-crystal panel 810 and blocking the rear side of the shutter film 820 in an indistinguishable manner. Accordingly, the liquid crystal display device 800 can visibly display the image displayed in the liquid-crystal panel 810 and can enable the rear side to be transparently visible through the liquid-crystal panel 810.
Regarding liquid crystal display devices having a function of a transparent display as described above, conventionally, liquid crystal display devices with one-screen display have been mainly developed. However, in the case of the one-screen display, there is a limit to the improvement in expressive power. Therefore, there is considered enhancing expressive power by displaying two superposed images using two liquid-crystal panels to enable display of images with a sense of depth and three-dimensional appearance. Hereinafter, a display device having such a configuration of two-ply display surface will be referred to as a “dual display”. It should be noted that, in Japanese Patent Application Laid-Open No. 2004-151186, an invention of such a dual display is disclosed.